Abstract: During the ultrasonic flaw detection process of the welding area of drill pipes in an oil drilling company, transversal cracks were found on the chamfer shoulder surface of a drill pipe. Through the macroscopic observation, metallographic examination, chemical composition analysis and energy spectrum analysis, the causes of transverse cracks on the chamfer shoulder surface of the drill pipe joint were analyzed. The results show that:the compositions of the joint material were not uniform, and serious metallurgical defects such as element segregation and looseness existed in the local region of the raw material, which destroyed the continuity of the matrix; in the subsequent manufacturing process, the metallurgical defects further extended, and finally led to the cracks. Related suggestions were put forward in order to avoid the loss caused by this kind of defects.